1. Field of the Invention
The present invention relates to an optical device suitable for use as a wavelength-multiplex optical terminal for demultiplexing a light beam in a particular wavelength range from signal light beams in various wavelength ranges (e.g., communication service signal or video signal light beams) transmitted through an optical fiber.
2. Description of the Related Art
With the recent development of wavelength-multiplex communications using a fiber amplifier, it has become customary to monitor the amounts of optical signals at respective wavelengths, adjust the amounts of optical signals, and then have an amplifier amplify the optical signals in order to maintain desired amplifier characteristics.
Various processes are known for monitoring the amounts of optical signals. According to the known processes, since monitor devices are associated with respective optical fibers, they are alone required to have a considerable size.
There have been demands for a small-size monitor device which is packaged with high density. A monitor device monitors an optical signal by extracting a portion of the optical signal. It is desirable for a monitor device to be able to monitor an optical signal without significantly attenuating the optical signal.
Heretofore, solutions to the above problems and demands have been proposed in Japanese Laid-Open Patent Publication No. 2003-294990, Japanese Laid-Open Patent Publication No. 2003-294992, Japanese Laid-Open Patent Publication No. 2003-295000, WO 03/096095 A1, WO 03/060584 A1 and WO 03/098293 A1.
According to FTTH (Fiber To The Home) services or the like, for example, since different upstream and downstream signal wavelengths are used, the subscriber needs to have a wavelength-multiplex optical terminal for processing upstream and downstream signals having different wavelengths. In the future, there will be a possibility to transmit a downstream video signal at another wavelength, and a three-wavelength wavelength-multiplex optical terminal will be required to handle all those signals.
The wavelength-multiplex optical terminal mainly comprises a WDM (Wavelength Division Multiplexing) filter, a light-detecting unit (e.g., a photodiode) for receiving a downstream signal, and a light-emitting unit (e.g., a laser diode) for transmitting an upstream signal.
For demultiplexing wavelengths with WDM filters, ordinary filters are required to have an attenuating level of about 25 dB in the cut-off range, but filters designed to meet special specifications are required to have an attenuating level of about 40 dB in the cut-off range. As shown in OPTCOM, March 2004, page 38, a bandpass filter (rejection filter) may be inserted between a WDM prism and a light-detecting unit.
Conventional wavelength-multiplex optical terminals are mainly classified into a microoptics design using a lens and a PLC design using an optical waveguide, as shown in OPTRONICS, January 2004, page 173.
Since both designs employ a light transmitting component (e.g., a lens or an optical waveguide) for guiding optical signals to a WDM filter, it is necessary to meet the most important requirement for optical components, i.e., optical alignment between the optical fiber and the light transmitting component. Accordingly, the cost of the resultant assembly tends to be high and the connection is liable to be of low reliability.